The present invention relates to fiber-reinforced liquid foams made from unsaturated polyester resins, suitable to be transformed into reinforced solid cellular materials having a density lower than 0.7 kg/l if said materials contain reinforcing fibers but not fillers, and lower than 1.0 kg/l, if inert fillers are also present.
The present invention also relates to a process for preparing said liquid foams essentially by mechanical incorporation of a gas or a mixture of gases into an unsaturated polyester resin reinforced with synthetic, vegetable or mineral fibers, containing conventional additives.
Further objects of the present invention will be described hereinafter.
Solutions of unsaturated polyester resins in styrene, reinforced with the reinforcing materials hereinbefore specified, are very difficult to transform into foams, inasmuch as only with difficulty do such solutions retain air or inert gases occluded in a stable and regular manner.
In order to overcome this drawback it has been proposed to employ chemical foaming agents such as e.g. isocyanates, certain azo-compounds, and so forth, which are added to the unsaturated polyester resin at the moment at which the foams are to be formed.
In practice, however, said agents have not yielded satisfactory results, because the copolymerization temperature of the solutions of unsaturated polyester resins in styrene begins to rise very slowly, so that the resin gels a long time before the temperature has become high enough to completely activate the foaming agent.
Further said processes have other disadvantages, such as e.g.:
the chemical agents employed as foaming agents are usually toxic and are generally unstable at room temperature and therefore must be maintained at low temperatures until they are used; and the process is not economical inasmuch as the chemical foaming agent is expensive.
It has also been proposed (U.S. Pat. Nos. 4,120,923 or 3,362,919) to dissolve, under pressure, in the unsaturated polyester resin, gases such as carbon dioxide or air, which, once the pressure is released, for example by means of a nozzle, will cause the resin to foam.
However this type of foaming known as "physical foaming" is not suitable for continuous industrial processes. If reinforcing fibers are present in the resin to be foamed, than the nozzle will become quickly occluded with such fibers and no foam will be produced.
According to another discontinuous method of physical foaming, volatile, low boiling, halogenated hydrocarbons such as CClF.sub.3, CHClF.sub.2 or CCl.sub.2 F.sub.2 are dissolved in the resin at a low temperature and foam is produced when the temperature is raised above the boiling point of the hydrocarbon.
The foam eventually produced by these methods has large gas bubbles which are not homogeneously distributed.